Which level 7 creature is the best, and why?

I still think Phoenixes are the best units because you need to consider the unit's actual combat characteristics, not ideal conditions (hero with max stats, one-on-one). The price/quality/growth ratio for this unit is optimal.

Added 9 minutes later
As for magic resistance, it can also negatively affect the outcome of the battle. Spells like restoration and haste are sometimes simply essential to use.